Spinal deformities, which include rotation, angulation, and/or curvature of the spine, can result from various disorders, including, for example, scoliosis (abnormal curvature in the coronal plane of the spine), kyphosis (backward curvature of the spine), and spondylolisthesis (forward displacement of a lumbar vertebra). Other causes of an abnormally shaped spine include trauma and spinal degeneration with advancing age. Early techniques for correcting such deformities utilized external devices that applied force to the spine in an attempt to reposition the vertebrae. These devices, however, resulted in severe restriction and in some cases immobility of the patient. Furthermore, current external braces have limited ability to correct the deformed spine and typically only prevent progression of the deformity. Thus, to avoid this need, doctors developed several internal fixation techniques to span across multiple vertebrae and force the spine into a desired orientation.
To fix the spine, surgeons attach one or more spinal connectors (typically rods or plates) to the spine at several fixation sites to correct and stabilize the spinal deformity, prevent reoccurrence of the spinal deformity, and stabilize weakness in trunks that results from degenerative discs and joint disease, deficient posterior elements, spinal fracture, and other debilitating problems. Bone screws are typically used to anchor the spinal connectors at the various fixation sites. Once anchored, the system is under stress and subjected to significant forces, known as cantilever pullout forces. As a result, surgeons are always concerned about the possibility of the implant loosening or the bone screws pulling out of the bone. Thus, surgeons generally seek to attach implants in the most secure and stable fashion possible while at the same time addressing a patient's specific anatomy.
Most current fixation procedures utilize two spinal connectors anchored along opposed lateral sides of the spinal column. Fixation at a single level of the spine typically requires four bone screws. As a result, two incisions are often made in each lateral side of the spine at each fixation level to provide access for inserting the bone screws. In order to avoid the need to create multiple incisions and to reduce the operative and post-operative recovery time, recent fixation trends have utilized unilateral fixation, where only one spinal connector is anchored at various fixation sites along one lateral side of the spine. Because the natural forces through the spine are centered down the middle of the spine, unilateral fixation constructs must be designed to counteract the offset forces. The excess stress applied to current unilateral constructs tend to weaken the bone screws, causing them to rotate within the pedicle, and can weaken the strength of the spinal connector itself.
Accordingly, there is a need in this art for improved methods and devices for unilateral fixation.